Well I can speculate based on some numbers, but there is no concrete numbers for this.
In the Attack of The Clones on Kamino Lama Su (a Kaminoan) says:
Two hundred thousand
units are ready, with another
million well on the way.
So at the time of the first battle for Geonosis The droid armies were outnumbered by only an army of 200,000 clones at the most. Examining the droid army we see they have the planet carrier ships (the big ball looking things trying to take off), each of which contains an invasion army. I'd imagine that one of those could be a "garrison" at least. So 200,000 clones vs 5 or so garrisons of droids resulted in the droids retreating quickly. So if we operate under the assumption a planetship sontains roughly 1 garrison I would make an educated guess and say the number of droids in a "garrison" would be around 50,000 droids.
EDIT: I'm assuming that the structure of the droid armies is similar during the second battle of Geonosis. If you consider the fact that clones take a very long time to make ( I read they had a production rate of about 1.8 mil/year http://scifi.about.com/od/starwarsglossaryandfaq/a/Star-Wars-Faq-How-Many-Clone-Troopers-Are-There.htm) , and the second for Geonosis was in the same year, so there was not a lot of time to produce more droids or clones, they did not have many more clones available at the time. So considering this matchup, I think that it is fair to assume that it is still around 50k per garrison unit.
TL;DR around 50,000.